Decrease in section thickness on exposure to the electron beam; the use of tilted sections in estimating the amount of shrinkage.

It has been known for some time that considerable changes in the physical properties of thin sections may be induced by irradiation in the electron microscope. There have been few systematic studies, however, of the nature of these changes, particularly when the sections contain biological material. Cosslett (1961) showed that thin sections of several embedding media commonly used in the electron microscopy of biological systems became thinner by a finite amount after exposure to the electron beam at normal viewing intensities. The change in thickness was between 20 and 50 %, depending on the embedding medium, and was found to be very rapid for the beam intensities used. It is also known that when supporting films and sections of embedding media are exposed to an electron beam much of the material evaporates and a residue containing a high proportion of carbon remains; in some cases more than half the original mass may be lost (Bahr, Johnson & Zeitler, 1965; Reimer, 1965). The loss of weight can be correlated with the decrease in section thickness. Recently Willis (1973) has shown that the thickness of a section of biological material changes dramatically on exposure to the electron beam at normal viewing intensities for high-resolution microscopy. By re-embedding a section, part of which had been irradiated, and then sectioning perpendicular to the original section, he showed that the irradiated area was only half as thick as the non-irradiated region. Such a change in section thickness will cause distortions in the 3-dimensional structure of the specimen under observation and although this will have little effect on measurements made on sections viewed normally, it is clear that measurements made on tilted sections will require correction. The use of goniometer stages for tilting sections has become a widely used technique for investigating 3-dimensional structures.